U.S. Pat. No. 6,196,973 discloses a method for determining a blood flow velocity in a patient's blood vessel, into which a contrast agent has been introduced. Reflectors such as microbubbles introduced into a blood vessel of a patient are destroyed by focused ultrasound at a modulation point, thereby creating gaps in the flowing contrast agent that are sensed downstream sensing point. The pattern of gaps is then matched in time with the input modulation sequence to determine a transit time for the gaps, which is also the flow velocity of the blood.